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also differ in rates or timings of growth and/or morphogenesis, so there are more than
just the three possibilities shown in
Figure 11.10
.
There are also at least four cases in which the ontogenies of shape are modified
(see
Figure 11.10D, E, F, G
). In the first case, the two trajectories differ solely in direction
the species are identical at the outset of development but progressively diverge over time
(see
Figure 11.10D
). There is no general term for this case, although one suggestion is
“allometric repatterning” (
Webster and Zelditch, 2005
). In the second case, the two trajec-
tories differ in the starting point as well as direction; they differ in shape at the outset of
the observed development but their ontogenetic trajectories point towards the same adult
shape so they converge on the same adult form (see
Figure 11.10E
). This has been termed
“ontogenetic convergence” (
Adams and Nistri, 2010
). Of course, the divergent trajectories
could also lead to divergent adult morphologies; the species could diverge further over
the course of development, increasing the distance between the ancestral and descen-
dant shapes. In the third case, the two trajectories differ in length plus direction
(see
Figure 11.10F
); the two species develop at different rates, in different directions. To
our knowledge, there is no term for this possibility. In the fourth case, the two species
differ in all three attributes: shape at the outset of the measured phase, direction and
length (see
Figure 11.10G
). This is another case for which, to our knowledge, there is no
term. Species that have complex, curving ontogenetic trajectories add to these possibilities.
At present, we do not know which of these possibilities occurs most frequently, nor do
we know which contributes most to disparity. Until fairly recently, most studies focused
on ontogenetic scaling, in particular, or heterochrony, more generally. As a result, these
two topics dominate the literature. But that does not mean that either is especially
common or that either makes a large contribution to disparity. One reason for focusing on
ontogenetic scaling was to find the traits that do not evolve by extending or truncating the
ontogenetic trajectory because such extensions or truncations were expected when body
size evolves; consequently, the traits that do not exhibit such extensions or truncations
were thought to require a specific, adaptive explanation. In that sense, ontogenetic scaling
simply served as a “criterion for subtraction” (
Huxley, 1932; Gould, 1966
). Heterochrony,
however, was seen as especially interesting, one that challenged conventional evolutionary
theory. Whether frequent or not, heterochrony was seen as worthy of special attention.
The reasons why Gould thought that heterochrony is especially interesting are important
for understanding his analytic scheme as well as its reformulation by Alberch and
colleagues. Gould's arguments about the theoretical meaning of heterochrony are
grounded in intuition rather than formal theory, but those intuitions motivated the fasci-
nation with heterochrony. As Gould construed “heterochrony”, it referred to the changes in
developmental rate and/or timing that produce the parallelism between ontogeny and phylogeny.
Because ontogenetic scaling is a special case, we include it in our discussion of heterochrony.
Heterochrony
Gould (1977)
devoted his entire topic on ontogeny and phylogeny to heterochrony
because he regarded it as especially interesting and as challenging to traditional evolution-
ary theory. The first reason why he regarded heterochrony as especially interesting is that
